This disclosure relates generally to distributed temperature sensing (DTS) systems and, more particularly, to methods and systems for extending the range of fiber optic DTS systems.
For several years, fiber optic sensors, and in particular, DTS systems, have provided higher bandwidth, inherently safe operation (no generation of electric sparks), and immunity from EMI (Electromagnetic Interference) for parameter measurements.
For example, the temperature profile parameter and other parameter profiles along the fiber can be monitored. The resulting distributed measurement is equivalent to deploying a plurality of conventional point sensors, which would require more equipment and increase operational costs. Each conventional electrical point sensor would require multiple electrical leads and this would add to a large and expensive cable bundle as the number of point sensors increase.
When an optical fiber is excited with a laser light having a center wavelength λ, most of the light is transmitted. However, small portions of incident light λ and other excited components are scattered backward and forward along the fiber. The amplitude of the other excited components depends on the intensity of the light at center wavelength λ and the properties of the optical fiber. In the measurement of distributed temperature using Raman scattering, three components are of particular interest. The three components are Rayleigh back-scattered light, which will have a similar wavelength λ as the original laser wavelength, Raman Stokes and Raman anti-Stokes components which have longer and shorter wavelengths than the original wavelength λ. These three components can be separated by optical filters and received by photo detectors to convert light to electrical signals. A ratio between the temperature sensitive Raman anti-Stokes intensity to the temperature insensitive Rayleigh or largely temperature insensitive Raman Stokes intensity forms the basis of a Raman based distributed temperature measurement.
One problem with current systems and techniques is the ability to measure these parameter profiles over an extended distance, where the optical signal tends to degrade due to the attenuation along the fiber. In conventional fiber optic Raman based DTS systems, as an example, when the intensity of the input light is increased, the Raman Stokes and Raman anti-Stokes respective power in the optical fiber increases as well. This phenomenon is called Spontaneous Raman Scattering. When the input power of the optical source is further increased above a threshold level, stimulated scattering may occur either due to Brillouin scattering or Raman scattering. Stimulated Brillouin scattering manifests itself through the generation of a backward propagating Brillouin Stokes wave that carries most of the input energy once the Brillouin threshold is reached. The threshold level depends on light source properties such as peak power and spectral width, and optical fiber properties such as chemical composition of the fiber, Numerical Aperture and mode field diameter. Once the Brillouin threshold is reached, increased backward propagating non-linear stimulated Brillouin Stokes light may saturate the detector while limiting the amplitude of the forward propagating light. For these reasons, increasing the light energy by increasing the laser power is not a viable approach to increasing the distance reach for a conventional DTS system as the increase in signal energy is back scattered. Stimulated Brillouin scattering is often what limits the maximum power that can be transmitted into optical fibers using narrow line-width high power lasers.
Similarly, stimulated Raman scattering transfers energy in a non-linear fashion from the center light wavelength λ to the Raman Stokes component. As a result, the ratio between Raman Stokes and Raman anti-Stokes varies without temperature changes, thus generating errors in temperature calculations. Data taken in the fiber length where non-linear stimulated interactions occur tends to generate significant errors in temperature calculations.
Hartog et al disclosed a scheme (U.S. Pat. No. 7,304,725) based on a sensing system composed of two sequential physically different fibers with different Numerical Apertures to avoid this effect. They also disclosed another system, in which an optical amplifier (more precisely a length of rare-earth doped fiber in a section of the sensing fiber) was placed in between two sensing fibers to boot up the attenuated input optic energy to reach further distance.
Such approaches introduce cost and complexity in both design and operation. Accordingly, systems and methods that provide for extending the range of fiber optic DTS systems without undue complexity in the sensing fiber design and deployment are desired.